The trend toward reductions in the size and thickness of implantable medical devices such as implantable cardioverter-defibrillators (ICDs) has led to the need for miniaturization of the electrochemical components utilized in such devices. Capacitors, for example, are employed in ICDs typically implanted in a patient's chest to treat very fast, and potentially lethal, cardiac arrhythmias. These devices continuously monitor the heart's electrical signals and sense if, for instance, the heart is beating dangerously fast. If this condition is detected, the ICD can deliver one or more electric shocks, within about five to ten seconds, to return the heart to a normal heart rhythm. These electrical stimuli may range from a few micro-joules to very powerful shocks of approximately twenty-five joules to forty joules.
Early generations of ICDs utilized high-voltage, cylindrical capacitors to generate and deliver defibrillation shocks. For example, standard wet slug tantalum capacitors generally have a cylindrically shaped conductive casing serving as the terminal for the cathode and a tantalum anode connected to a terminal lead electrically insulated from the casing. The opposite end of the casing is also typically provided with an insulator structure.
One such capacitor includes a metal container that functions as a cathode. A porous coating, including an oxide of a metal selected from a group consisting of ruthenium, iridium, nickel, rhodium, platinum, palladium, and osmium, is disposed proximate an inside surface of the container and is in electrical communication therewith. A central anode selected from the group consisting of tantalum, aluminum, niobium, zirconium, and titanium is spaced from the porous coating, and an electrolyte within the container contacts the porous coating and the anode.
While the performance of these capacitors was acceptable for defibrillator applications, efforts to optimize the mechanical characteristics of the device have been limited by the constraints imposed by the cylindrical design. In an effort to overcome this, flat electrolytic capacitors were developed. One such capacitor comprises a deep-drawn sealed capacitor having a generally flat, planar geometry. The capacitor includes at least one electrode provided by a metallic substrate in contact with a capacitive material. The coated substrate may be deposited on a casing side-wall or connected to a side-wall. The capacitor has a flat planar shape and utilizes a deep-drawn casing comprised of spaced apart side-walls joined at their periphery by a surrounding intermediate wall. Cathode material is typically deposited on an interior side-wall of the conductive encasement which serves as the negative terminal for the electrolytic capacitor, though such material may also be deposited on a separate substrate and electrically coupled to the capacitor encasement. The other capacitor terminal (i.e. the anode) is isolated from the encasement by an insulator or feedthrough structure including, for example, a glass-to-metal seal. In accordance with one known technique, an anode lead (e.g. tantalum) imbedded into the anode is laser welded to a terminal lead that passes through the ferrule. This anode lead-to-feedthrough terminal weld joint (i.e. cross-wire weld) is formed by shaping one or more of the leads into a “U” or “J” shape, pressing the terminal ends of the leads together, and laser welding the interface.
A valve metal anode made-from metal powder is pressed and sintered to form a porous structure, and a wire (e.g. tantalum) is imbedded into the anode during pressing to provide a terminal for joining to the feedthrough. A separator (e.g. polyolefin, a fluoropolymer, a laminated film, non-woven glass, glass fiber, porous ceramic, etc.) is provided between the anode and the cathode to prevent short circuits between the electrodes. Separator sheets are sealed either to a polymer ring that extends around the perimeter of the anode or to themselves.
A separate weld ring and polymer insulator may be utilized for thermal beam protection as well as anode immobilization. Prior to encasement welding, a separator encased anode is joined to the feedthrough wire by, for example, laser welding. This joint is internal to the capacitor. The outer metal encasement structure is comprised essentially of two symmetrical half shells that overlap and are welded at their perimeter seam to form a hermetic seal. This weld is referred to as a rotary weld since the part is welded as it rotates on its side rather than employing a top-down approach. Alternatively, a top-own approach may be utilized to weld a lid onto a deep-drawn container. After welding, the capacitor is filled with electrolyte through a port in the encasement.
The above described techniques present concerns relating to both device size and manufacturing complexity. The use of overlapping half-shields results in a doubling of the encasement thickness around the perimeter of the capacitor thus reducing the available interior space for the capacitor's anode. This results in larger capacitors. Space for the anode material is further reduced by the presence of the weld ring and space insulator. In addition, manufacturing processes become more complex and therefore more costly, especially in the case of a deep-drawn encasement.
The abovementioned method of joining an anode lead to a terminal lead was found to be problematic, however, as the step of cross-wire welding must be performed prior to welding the feedthrough ferrule to the capacitor encasement or sufficient space must be provided in the capacitor anode structure to facilitate clamping and welding following ferrule welding. Producing the cross-wire weld prior to ferrule welding subjects the materials employed in the feedthrough seal to thermal stress and increases the cost and complexity of manufacture. Conversely, performing cross-wire welding after ferrule welding has a negative impact on volumetric efficiency.
As mentioned above, it is common for the anode terminal to be isolated from the encasement by an insulator or feedthrough structure comprised including a glass-to-metal seal. Such seals are well known in the art. To avoid problems which may be encountered due to the rigidity of glass-to-metal seals, polymer-to-metal seals have been employed. For example, it is known to secure an anode lead within a ferrule by means of a series of polymeric sealing layers. These layers may comprise a first layer of a synthetic polymeric material forming a plug on end of the ferrule internal to the electrolytic cell, a second layer of synthetic polymeric material disposed within the ferrule, and a third layer of glass disposed within the ferrule to provide a hermetic seal. Similar assemblies, varying in the arrangement and/or shape of the polymeric layers, are also known. Unfortunately, current methods of manufacturing such assemblies are relatively complex, time-consuming, and expensive.
It should thus be appreciated that it would be desirable to provide an electrochemical device including an improved feedthrough assembly that is volumetrically efficient and simple to manufacture.